An oxymoron (Greek oxumōros, pointedly foolish, from mōros, foolish) is a figure of speech in which apparently contradictory terms appear in conjunction, such as faith unfaithful; oxytocin (Greek tokos, childbirth) is a hormone released by the pituitary gland during labour. The Greek root is also the origin of oxygen, originally thought to be essential for the formation of all acids (see the next entry for terms in oxy- involving oxygen), and oxalic acid, named via Latin from Greek oxalis, wood sorrel, because of its sharp-tasting leaves that contain the acid.